Synthesis of peptides

ABSTRACT

New peptides are disclosed which have biological activity of the same type as known calcitonins and which have a shorter amino acid chain than natural calcitonins. Also resin peptides are disclosed which may be converted to peptides having such biological activity; and processes for producing said resin peptides and said calcitonin peptides.

FIELD OF INVENTION

This invention relates to calcitonins having biological activity and to peptides which can be converted to biologically active calcitonins and to processes for preparing such peptides and calcitonins.

BACKGROUND OF THE INVENTION

All known natural calcitonin peptides contain an amino acid sequence of 32 amino acids. Salmon calcitonin, for example, has the following formula: ##STR1##

In U.S. Pat. Nos. 3,926,938, 4,062,815, 3,929,758, 4,033,940 and 4,217,268 are disclosed improved syntheses of calcitonins including the salmon calcitonin above referred to.

The natural calcitonins include the salmon calcitonins, bovine, porcine, ovine, and eel calcitonins and human calcitonins. All of these have leucine at position number 16 except human calcitonin which has phenylalanine at position number 16.

SUMMARY OF THE INVENTION

We have discovered synthetic calcitonin peptides with 31 amino acids that have relatively high competitive binding ability and low hypocalcemic activity. A significant difference in structure is that in our new peptides the amino acid sequence does not contain the leucine or phenylalanine residue at position 16 of the known calcitonin amino acid sequence. This new peptide is termed des-X¹⁶ -calcitonin where X is leucine or phenylalanine, using the IUPAC-IUB method of nomenclature for synthetic modifications of peptides [Biochem. Biophys. Acta, 133, 1-5 (1967)]. The new peptides have good potency and quality when compared to known calcitonins.

DESCRIPTION OF INVENTION

In general we use a solid phase type of synthesis and start with a resin called benzhydryl amine resin (BHA resin). This resin is derived from a cross-linked polystyrene bead resin manufactured by copolymerization of styrene and divinylbenzene. Resin of this type is known and its preparation is further demonstrated by Pietta et al [Pietta, P. S. and Marshall, G. R., Chem. Commun., 650 (1970)]. This cross-linked polystyrene BHA resin is available from chemical supply houses. We use the designation ##STR2## to represent the BHA resin in which R is the polystyrene portion of the resin.

Resin Peptide Synthesis

In this synthesis the amino acids are added one at a time to the insoluble resin until the total peptide sequence has been built up on the resin. The functional groups of the amino acids are protected by blocking groups. The α-amino group of the amino acids is protected by a tertiary butyloxycarbonyl group or an equivalent thereof. This α-tertiary butyloxycarbonyl group we designate as BOC. The hydroxyl functions of serine and threonine are protected by a benzyl or benzyl derivative group such as 4-methoxybenzyl, 4-methylbenzyl, 3,4-dimethylbenzyl, 4-chlorobenzyl, 2,6-dichlorobenzyl, 4-nitrobenzyl, benzhydryl or an equivalent thereof. We use the term BZ to represent the benzyl or benzyl derivative group.

The hydroxyl function of tyrosine may be unprotected, may be protected by a benzyl or benzyl derivative group as described above, by a BZ group, or may be protected by a benzyloxycarbonyl or a benzyloxycarbonyl derivative such as a 2-chlorobenzyloxycarbonyl or 2-bromobenzyloxycarbonyl group or equivalent thereof. We use the term W to represent either no protective group, a BZ group, a benzyloxycarbonyl group or a benzyloxycarbonyl derivative group.

The thiol function of cysteine may be protected by benzyl or benzyl protective groups described above and designated BZ or by an n-alkylthio group such as methylthio, ethylthio, n-propylthio, n-butylthio or equivalents thereof. We use the character R₂ to represent an n-alkylthio group or BZ and the character R₁ to represent BZ when R₂ is n-alkylthio and to represent n-alkylthio when R₂ is BZ. Alternatively, R₁ may be another cysteine group and when this is the case R₂ is BZ. The quanidine function of arginine may be protected by a nitro group, a tosyl group or an equivalent thereof. We use the character T to represent either a nitro group or a tosyl group. The ε-amino function of lysine may be protected by a benzyloxycarbonyl group or a benzyloxycarbonyl derivative such as 2-chlorobenzyloxycarbonyl, 3,4-dimethylbenzyloxycarbonyl, or equivalents thereof. We use the character V to represent a benzyloxycarbonyl group or a benzyloxycarbonyl derivative group. The protective groups used on the imidazole nitrogen of histidine are the benzyloxycarbonyl group and benzyloxycarbonyl derivatives such as described above for lysine and are designated as V. The γ-carboxylic acid group of glutamic acid is protected by a benzyl or benzyl derivative group such as described for the protection of the hydroxyl function of serine and threonine. These protective groups are represented by the character BZ.

The formula of our new Des-Leu¹⁶ -salmon calcitonin having activity of the same type as known salmon calcitonin may be written as follows: ##STR3##

The formula of our new Des-Leu¹⁶ -eel calcitonin may be written as follows: ##STR4##

The formula of our new Des-Phe¹⁶ -human calcitonin may be written as follows: ##STR5##

As may be seen from the formula above given, 31 amino acids are involved and in this formula the positions are numbered according to the accepted procedure beginning at position 1 for the CYS on one end of the chain, omitting position 16 for the missing leucine in the sequence and ending with PRO at position 32 at the other end of the chain. For clarity of description, this same numbering system will be followed in referring to the cycles of the synthesis. The assembly of the amino acids begins with cycle 32 which involves the coupling of proline and continues with cycle 31 which involves the coupling of threonine, etc.

Preferred amino acid reactants for use in each of the 31 cycles of the synthesis are given in the following Table I:

                  TABLE I                                                          ______________________________________                                         Cycle                                                                          Num-                                                                           ber   Amino Acid Reactant                                                      ______________________________________                                         32    BOC--L-proline                                                           31    BOC--O--benzyl-L-threonine                                               30    BOC--glycine                                                             29    BOC--O--benzyl-L-serine                                                  28    BOC--glycine                                                             27    BOC--O--benzyl-L-threonine                                               26    BOC--L-asparagine p-nitrophenyl ester                                    25    BOC--O--benzyl-L-threonine                                               24    BOC--Ω-nitro-L-arginine or BOC--Ω-tosyl-L-arginine           23    BOC--L-proline                                                           22    BOC--O--benzyl-L-tyrosine, BOC--L-tyrosine,                                    or BOC--O--2 bromobenzyloxycarbonyl-L-tyrosine                           21    BOC--O--benzyl-L-threonine                                               20    BOC--L-glutamine p-nitrophenyl ester                                     19    BOC--L-leucine                                                           18    BOC--ε-CBZ--L-lysine or BOC-ε-2-chloro-                        benzyloxycarbonyl-L-lysine                                               17    BOC--N(im)--CBZ--L-histidine                                             15    BOC--L-glutamic acid γ-benzyl ester                                14    BOC--L-glutamine p-nitrophenyl ester                                     13    BOC--L-benzyl-L-serine                                                   12    BOC--L-leucine                                                           11    BOC--ε-CBZ--L-lysine or BOC--ε-2-chloro-                       benzyloxycarbonyl-L-lysine                                               10    BOC--glycine                                                             9     BOC--L-leucine                                                           8     BOC--L-Valine                                                            7     BOC--S--ethylthio-L-cysteine, BOC--S--methylthio-                              L-cysteine, BOC--S--n-propylthio-L-cysteine or                                 BOC--S--n-butyltho-L-cysteine                                            6     BOC--O--benzyl-L-threonine                                               5     BOC--O--benzyl-L-serine                                                  4     BOC--L-leucine                                                           3     BOC--L-asparagine p-nitrophenyl ester                                    2     BOC--O--benzyl-L-serine                                                  1     BOC--S--p-methoxybenzyl-L-cysteine, BOC--S--ben-                               zyl-L-cysteine or BOC--S-- 3,4-dimethylbenzyl-                                 L-cysteine, Bis Boc Cystine                                              ______________________________________                                    

Each of the amino acid derivatives mentioned in Table I may be purchased from supply houses.

CYCLE 32 Coupling Of Proline To BHA Resin

The reaction vessel used in all steps of the resin peptide synthesis may be a glass vessel equipped with inlet ports at the top for addition of materials and a sintered glass disk at the bottom for removal of soluble reaction mixtures and wash solvents by filtration. Filtration may be performed either by vacuum or the use of nitrogen pressure. The contents of the vessel may be agitated by shaking the entire vessel or by mechanical stirrer.

In cycle 32 the BHA resin is placed in the reaction vessel and suspended in a solvent such as methylene chloride, chloroform, dimethylformamide, benzene or equivalents thereof in proportions of about 3 to 12 ml. of solvent per gram of resin. To this is added BOC-L-proline in an amount of about 1 to 6 equivalents per free amine equivalent of the BHA resin employed. After a period of mixing of 5 to 10 minutes, a coupling reagent (CA) such as dicyclohexyl carbodiimide (DCC) may be added, or other diimide coupling agents may be used. The diimide coupling agent may be used in the amount of 0.5 to 2.0 equivalents per equivalent of BOC-L-proline used.

The BOC-L-proline may be coupled in the absence of a coupling reagent if its active ester derivative, its azide derivative, its symmetrical anhydride derivative, or a suitably chosen mixed anhydride derivative is used. Active ester derivatives that may be employed are 2-nitrophenyl ester, 4-nitrophenyl ester, pentafluorophenyl ester, N-hydroxysuccimide ester or equivalents thereof. The active esters are used in amounts of 1 to 10 equivalents per free amine equivalent of BHA resin.

The reaction mixture consisting of the BHA resin, the solvent, the BOC-L-proline, and the coupling reagent or BOC-L-proline active ester is stirred or shaken mechanically until the reaction is complete as indicated by a ninhydrin test [E. Kaiser, et al., Anal. Biochem., 34, 595-8 (1970)]on a test sample. After completion of the coupling reaction, the BOC-L-proline resin may be washed with solvents such as methylene chloride, chloroform, methyl alcohol, benzene, dimethylformamide, or acetic acid. The amount of wash solvent may suitably be 5 to 20 ml. of solvent for each gram of BHA resin used initially. If it is desired to terminate the coupling reaction before completion, the washing procedure may be used and the remaining free amino groups on the BOC-L-proline resin may be blocked from further reaction by acetylation with an excess of acetylation reagents. The acetylation procedure may be performed by agitating the BOC-L-proline resin with a solution of the acetylation reagent for a period of 0.5 to 12 hours. Acetylation reagents such as N-acetylimidazole in methylene chloride solution or a mixture of acetic anhydride and triethylamine in chloroform may be used. The acetylation reagent may be used in the amount of 0.5 to 5.0 equivalents per equivalent of free amine titer of the starting BHA resin.

The coupling reaction to produce the BOC-L-proline resin may be described by the following formula: ##STR6##

Deprotection Of BOC-L-Proline Resin

The BOC-L-proline resin produced as above described may be washed with a solvent such as referred to above and deprotected by agitating it with an agent such as a mixture of trifluoroacetic acid (TFA) in a solvent such as methylene chloride, chloroform, benzene or equivalents thereof. The amount of TFA in the solvent may vary from 10 to 100% of the mixture. The amount of TFA-solvent mixture may vary from 3 to 20 ml. per gram of BHA resin used initially. The reaction time may be from about 10 minutes to 4 hours. The deprotection step is terminated by filtration to remove the TFA-solvent mixture. The residual TFA may be removed from the L-proline resin by washing with 3 to 20 ml. per gram of BHA resin of solution of 5 to 30% of triethylamine in a solvent such as methylene chloride, chloroform, benzene or equivalents thereof. Other tertiary or secondary organic amines may be used in place of the triethylamine, such as trimethylamine, N-ethylpiperidine, diisopropylamine or equivalents thereof. The free amine titer of the L-proline resin may be determined by the Dorman titration procedure (Dorman, L. C., Tetrahedron Letters, 1969, 2319-21). The deprotection reaction may be described by the following formula: ##STR7##

CYCLE 31

The prolyl BHA resin obtained as a result of cycle 32 may be suspended in a coupling solvent, the BOC-O-BZ-L threonine derivative added and the mixture equilibrated in the same manner. The coupling agent, DCC, may be added, and after completion of the reaction as indicated by the ninhydrin test, the reaction mixture may be removed from the BOC-O-BZ threonylprolyl BHA resin by filtration. The peptide resin may be washed with solvents. The amounts of reactants and solvents and reaction times may be the same as described in cycle 32. The BOC group may be removed from the peptide resin by the deprotection method described in the cycle 32. The resulting O-BZ-threonylprolyl BHA resin is then ready for cycle 30. The reactions of the cycle 31 may be shown by the following formula: ##STR8##

For convenience, we may write this resulting resin peptide using abbreviated nomenclature as follows: ##STR9##

CYCLE 30

In cycle 30, the coupling reaction and also the deprotection reaction may be performed in the same manner as in cycle 31 except that BOC-glycine is used in place of BOC-O-BZ-L-threonine. The reaction through coupling and deprotection may be written: ##STR10##

CYCLE 29

In cycle 30, the coupling and deprotection reactions may be performed in the same manner as in cycle 31 except for the substitution of BOC-O-BZ-L-serine as the amino acid derivative. This may be written. ##STR11##

CYCLE 28

In cycle 28, the coupling and deprotection reactions are performed as described in cycle 31 except that BOC-glycine is substituted as the amino acid reactant. These reactions through coupling and deprotection may be written as follows: ##STR12##

CYCLE 27

In this cycle, the coupling and deprotection reactions may be as in cycle 31 using the same amino acid reactant, resulting in the following compound: ##STR13##

CYCLE 26

In cycle 26, the coupling reaction is performed using an active ester derivative of BOC-L-asparagine. The active ester procedure is used in place of the DCC coupling agent with BOC-asparagine or BOC-glutamine. The reaction may be performed using the active ester derivative of BOC-L-asparagine in the amount of 2 to 10 equivalents per free amine equivalent of BHA resin in dimethylformamide, mixtures of dimethylformamide with benzene, methylene chloride or chloroform or with equivalents thereof in amount of 2 to 20 ml. of solvent per gram of BHA resin used initially.

Reaction times range from 1 to 72 hours. The reaction mixture may be removed from the BOC peptide resin by filtration after completion of the reaction as indicated by a ninhydrin test. The active ester derivatives employed may be 2-nitrophenyl esters, 4-nitrophenyl esters, pentafluorophenyl, or equivalents thereof. We use AE to designate the active ester portion of the derivative. The coupling reaction may be written: ##STR14##

The deprotection reaction to remove the BOC group is performed as in cycle 32.

CYCLES 25-21

In each of cycles 25 to 21, the coupling and deprotection reactions may be conducted using the methods and proportions of reactants as in cycle 31, using BOC-BZ-L-threonine in cycle 25, BOC-ω-T-L-arginine in cycle 24, BOC-L-proline in cycle 23, BOC-O-BrZ-L-Tyrosine in cycle 22, and BOC-O-BZ-L-threonine in cycle 21. The compound resulting from the completion of cycle 21 may be written. ##STR15##

CYCLE 20

In cycle 20, the coupling and deprotection reactions may be performed using the methods and proportions of reactants as in cycle 26 using a BOC-L-glutamine active ester derivative as the amino acid derivative, resulting in the compound: ##STR16##

CYCLE 19

In cycle 19, the reactions are performed as in cycle 31 using BOC-L-leucine as the amino acid derivative. The compound resulting from cycle 19 is: ##STR17##

CYCLE 18

In cycle 18, we may use as the amino acid derivative BOC-ε-Z-lysine. Otherwise, cycle 18 methods may be performed as in cycle 31 resulting in the compound: ##STR18##

CYCLES 17-15

Cycles 17 and 15 may be performed as in cycle 31 except for the use of BOC-N(im)-Z-L-histidine in cycle 17, BOC-L-glutamic acid BZ ester (BZ represents the same groups as it represents for serine and threonine) as the reactant in cycle 15, resulting in the following compound from cycle 15: ##STR19##

CYCLES 14-8

Cycle 14 may be performed identically to cycle 20 using BOC-L-glutamine-AE as the amino acid derivative. Cycles 13 to 8 may be performed as in cycle 31 except for the use of BOC-O-BZ-L-serine in cycle 13, BOC-L-leucine in cycle 12, BOC-ε-Z-L-lysine in cycle 11, BOC-glycine in cycle 10, BOC-L-leucine in cycle 9, and BOC-L-Valine cycle 8 resulting in the compound: ##STR20##

CYCLE 7

Cycle 7 may be performed as in cycle 31 except for the use of BOC-S-L-cysteine or for the amino acid derivative. The compounds resulting from cycle 7 are described by the formula: ##STR21## where R₂ is an alkylthio or a BZ group.

CYCLES 6-2

Cycles 6 to 4 may be performed as in cycle 31 except that BOC-O-BZ-L-threonine be used as the amino acid derivative in cycle 6, BOC-BZ-L-serine may be used as the amino acid derivative in cycle 5 and cycle 2, and BOC-L-leucine may be used in cycle 4 as the amino acid derivative. Cycle 3 may be performed identically to cycle 26 using BOC-L-asparagine active ester. The compound resulting from cycle 2 is: ##STR22##

CYCLE 1

This cycle may be performed identically to cycle 7 using BOC-S-R-L-cysteine derivatives. The R group chosen for the cysteine may be the same as used in cycle 7 or different. For example, if the derivative chosen for cycle 7 is BOC-S-ethylthio-L-cysteine, the derivative in cycle 1 may be BOC-S-4-methoxybenzyl-L-cysteine or if BOC-S-4-methoxybenzyl-L-cysteine was chosen for cycle 7, then this derivative may be used also in cycle 1. The compounds resulting from cycle 1 are illustrated by the formula: ##STR23## where R₁ is S-n-alkyl, CYS or Bz and R₂ is S-n-alkyl or Bz, R₁ being alkyl or CYS when R₂ is Bz and R₂ being Bz when R₁ is S-n-alkyl or CYS.

Cycle 1 represents the completion of the resin peptide, the resin peptide may be removed from the reaction vessel and dried in a vacuum. The weight of the resin peptide may be expected to be from 2.0 to 4.0 times weight of BHA resin used initially in the synthesis.

Resin Peptide Cleavage

The peptide is cleaved from the resin peptide resulting from cycle 1 by treatment with liquid hydrogen fluoride (HF). The HF cleavage reaction may be performed by treating a mixture of the resin peptide and anisole (0.5 to 5 ml. for each gram of resin peptide) with liquid HF (2 to 20 ml. for each gram of resin peptide) for 0.5 to 20 hours at -20 degrees to +15 degrees centigrade. After the reaction period, the excess HF may be removed by evaporation and the resulting mixture of peptide and resin beads may be extracted with organic solvent such as ethyl acetate, diethyl ether, benzene or the like to remove the anisole and residual of HF. The peptide may be separated from the resin beads by extraction into aqueous acetic acid. The peptide at this stage is not cyclic but is the non-cyclic product without the cyclic disulfide bond between the cysteines at positions 1 and 7 in the molecule.

The HF treatment removes all blocking groups from the peptide, except the S-alkylthio blocking groups on the thiol function of cysteine residue at either position 1 or 7. The S-n-alkylthiol-L-cysteine residue is stable to the HF cleavage procedure and remains intact throughout the cleavage and extraction procedures. The S-BZ-L-cysteine residue is cleaved by HF to yield a cysteine residue with a free thiol function. Both types of blocking groups have been employed during our synthesis in combination with each other at positions 1 and 7.

Thus, the peptides obtained after HF cleavage can be one of three types depending upon the blocking groups chosen for the thiol function of the cysteine derivative used during the resin peptide synthesis.

If BOC-S-BZ-L-cysteine derivatives are used in the resin peptide synthesis cycle 1 and BOC-S-n-alkylthio-L-cysteine is used in cycle 7, the peptide resulting after HF cleavage would be of Type I and would have a free thiol function at position 1 and have a S-n-alkylthio function on the cystine residue at position 7. We call this a Type I peptide which is represented by the formula given as follows: ##STR24##

Conversely, if BOC-S-n-alkylthio-L-cysteine derivative is used in cycle 1 and the BOC-S-BZ-L-cysteines were used in position 7, the peptide resulting from the cleavage would be of Type II and would be represented by the formula: ##STR25##

In place of the protecting group S-n-alkyl at position 1 we may use a cysteine group (which with the cysteine at this position forms a cystine group) and when we do this we use a BZ group for protecting the cysteine at position 7. If Bis-BOC-L-cysteine is used as the reactant in cycle 1 and the BOC-S-BZ-L-cystine is used as the reactant in cycle 7, the peptide resulting from the cleavage will be of Type III and may be represented by the following formula: ##STR26##

If BOC-BZ-L-cysteine is used as the reactant in both positions 1 and 7, the peptide resulting from the cleavage will be of Type IV and represented by the following formula:

H-CYS-SER-ASN-LEU-SER-THR-CYS-VAL-LEU-GLY-LYS-LEU-SER-GLN-GLU-HIS-LYS-LEU-GLN-THR-TYN-PRO-ARG-THR-ASN-THR-GLY-SER-GLY-THR-PRO-NH₂

The conversion of Types I, II and III peptides to the cyclic disulfide peptides may be performed by diluting with distilled water the aqueous acetic acid solution of the crude peptides from HP cleavage to a final volume of 50 to 200 ml. per gram of resin peptide cleaved. The pH of this solution may be adjusted to 5 to 10 by the addition of ammonium hydroxide solution and the mixture may be stirred in a closed container under a stream of an inert gas such as nitrogen for about 2 to 48 hours. The reaction period can be stopped when the off-gas stream no longer contains n-alkylmercaptan. The pH of the reaction mixture may be lowered to about 3.5 to 5.5 by the addition of glacial acetic acid.

The conversion of Type IV peptides to the cyclic disulfide peptide may be performed by the classical method known to the art in which the peptides are oxidized to join the ring structure to include in the ring the cystines at positions 1 and 7.

Whether the intermediate peptides are of Type I, II, III or IV we may synthesize peptides having amino acid chains corresponding to any known calcitonin except for the deletion of leucine or phenylalanine at position 16, and such peptide synthesized as herein set forth may be purified and found to have the same type of biological activity as this known calcitonin. Any calcitonin so synthesized is designated des-(X)¹⁶ -calcitonin. This is in accordance with the IUPAC-IUB method of nomenclature.

Purification Of The Crude Des Leu¹⁶ -SCT

The crude peptide solutions at pH 5.0 from the above synthesis may be concentrated using an ion-exchange procedure. The concentrate may be purified by a combination of gel-filtration procedures and ion-exchange chromatography methods. The final purified product may be obtained from solution by freeze-drying as a fluffy white solid. The product gives the correct amino acid analysis for the desired peptide.

Following is the specific example of the preparation of the peptide.

EXAMPLE 1 Resin Activation

The BHA resin (5 g.) with an amine titer of 0.61 meq./g. was placed in the reactor vessel of a peptide synthesizer marketed by Schwarz-Mann, Inc. of Orangeburg, New York. The resin was treated with 25 ml. of the following solvents filtering after each treatment:

Methylene chloride for 2 minutes

Chloroform for 2 minutes two times each

10% triethylamine in chloroform for 5 minutes two times each

Chloroform for 2 minutes

Methylene chloride for 2 minutes three times each

Cycle 32

Coupling: The BHA resin, 25 ml. of methylene chloride and 2.58 g. (0.012 moles) of BOC-L-proline was stirred for 10 minutes. 12.0 ml. of methylene chloride solution of dicyclohexylcarbodiimide (1 milliequivalent of DCCI per 1 ml. of solution) was added to the reactor and the mixture agitated for 6 hours. The reaction mixture was removed from the reactor by filtration and the BOC-prolyl BHA resin subjected to the following successive 2 minute, 25 ml. washes, removing the wash by filtration each time:

Methylene chloride two times

Methyl alcohol two times

Methylene chloride three times

Acetylation: The resin was then agitated with a mixture of 1.5 ml. of triethylamine (TEA), 1 ml. of acetic anhydride and 25 ml. of chloroform for 2 hours. The reaction mixture was removed by filtration and the resin subjected to the following 2 minute, 25 ml. washes:

Chloroform two times

Methyl alcohol two times

Methylene chloride three times

Deprotection: The BOC-protected resin was agitated for 5 minutes with a mixture of 15 ml. of trifluoroacetic acid (TFA) and 15 ml. of methylene chloride. This mixture was removed by filtration and the resin was agitated with a second mixture of 15 ml. of TFA and 15 ml. of methylene chloride for 30 minutes. The reaction mixture was removed by filtration and the resin subject to the following 25 ml. washes:

Methylene chloride two times two minutes each

Methyl alcohol two times two minutes each

Chloroform two times two minutes each

10% TEA in chloroform two times ten minutes each

Chloroform two times two minutes each

Methylene chloride two times two minutes each

The L-proline BHA resin was titrated to establish the amine or proline titer. This value was 0.55 milliequivalents of amine or proline per gram of resin.

Cycle 31

Coupling: The L-prolyl resin, 25 ml. of methylene chloride and 3.4 g. (0.0₁₁ mole) of BOC-O-benzyl-L-threonine were agitated for 10 minutes. Then 11.0 ml. of methylene chloride solution of dicyclohexylcarbodiimide (1 milliequivalent of DCCI per 1 ml. of solution or a total of 0.0¹¹ mole of DCCI) was added to the reactor and the mixture agitated for 2 hours. The reaction mixture was removed from the reactor and the resin was subjected to the following successive 2 minute, 25 ml. washes, removing the wash by filtration each time.

Methylene chloride two times

Methyl alcohol two times

Methylene chloride three times

A ninhydrin test was negative.

Deprotection: The deprotection procedure described in cycle 32 was repeated for this cycle.

Cycles 30 Through 27

The coupling and deprotection procedures used in these cycles were the same as in cycle 31 except that the following amino acid derivatives were used in place of the threonine derivative:

Cycle 30--1.92 g. (0.011 mole) of BOC glycine

Cycle 29--3.25 g. (0.011 mole) of BOC-O-benzyl-L-serine

Cycle 28--The reactant used was the same as cycle 30

Cycle 27--The reactant used was the same as cycle 31

Cycle 26

Coupling: The peptide resin obtained from cycle 27 was washed twice with 25 ml. portions of dimethylformamide (DMF). The resin was then agitated for 24 hours with a solution of 5.82 g. (0.0165 mole) of BOC-L-asparagine p-nitrophenyl ester in 35 ml. of DMF. The reaction mixture was filtered and the resin peptide subjected to two minute washes with two successive 25 ml. portions of the following solvents: DMF, methylene chloride, methanol, methylene chloride. Each individual solvent was removed by filtration. A ninhydrin test was negative.

Deprotection: The deprotection procedure used in cycle 32 was repeated.

Cycle 25

Coupling and deprotection procedures were the same as in cycle 31 using the same reactants and amounts.

Cycle 24

Coupling: The resin peptide obtained from cycle 25 was washed with two successive 25 ml. portions of DMF. The resin peptide was then agitated for 10 minutes with a mixture of 7.06 mg. (0.0165 mole) of BOC-N-y-tosyl-L-arginine and 25 ml. of DMF. Then 16.5 ml. of DCCI in methylene chloride (equivalent to 0.0165 mole of DCCI) was added and the mixture agitated for 6 hours. The reaction mixture was removed by filtration. The resin peptide was subjected to two minute washes with two successive 25 ml. portions of the following solvents: DMF, methylene chloride, methyl alcohol, methylene chloride. The ninhydrin test was negative.

Deprotection: The deprotection used in cycle 32 was repeated.

Cycle 23

Coupling: The peptide resin obtained from cycle 24 was agitated for 10 minutes with 3.54 g. (0.0165 mole) of BOC-L-proline and 25 ml. of methylene chloride. 16.5 ml. of DCCI in methylene chloride (equivalent to 0.0165 mole of DCCI) was added and the mixture agitated for 6 hours. The reaction mixture was removed by filtration and the resin peptide subjected to two minute washes with two successive 25 ml. portions of the following solvents: methylene chloride, methyl alcohol, methylene chloride. Each individual wash was removed by filtration. The ninhydrin test was negative.

Deprotection: The deprotection used in cycle 32 was repeated.

Cycles 22 and 21

The coupling and deprotection procedures used in these cycles were the same as in cycle 23 except that in the coupling reaction the following amino acid derivative was used in place of BOC-L-proline.

Cycle 22--8.15 g. (0.0165 mole) BOC-W-L-tyrosine

Cycle 21--5.1 g. (0.0165 mole) of BOC-O-benzyl-L-threonine

Cycle 20

This procedure is the same as cycle 26 except that 6.05 g. (0.0165 mole) of BOC-L-glutamine p-nitrophenyl ester is used in place of the asparagine derivative.

Cycles 19 through 15

The procedure is the same as used in cycle 31 except that the following amino acid derivatives were used in place of the threonine derivative:

Cycle 19--2.54 g. (0.011 mole) of BOC-L-leucine

Cycle 18--4.55 g. (0.011 mole) of BOC-ε-2-chlorocarbonzyloxy-L-lysine

Cycle 17--5.14 g. (0.011 mole) of BOC-N(im)-carbobenzyloxy-L-histidine

Cycle 15--3.70 g. (0.011 mole) of BOC-L-glutamic acid-γ-benzyl ester

Cycle 14

Same as cycle 20.

Cycle 13

The procedure used was the same as was used in cycle 23 except that in the coupling reaction 4.86 g. (0.0165 mole) of BOC-O-benzyl-L-serine was used in place of the proline derivative.

Cycles 12 through 9

The procedures used were the same as used in cycle 31 except in the coupling reactions the following amino acid derivatives were used in place of the threonine derivative.

Cycle 12--Same reactants as used in cycle 19

Cycle 11--The reactants were the same as in cycle 18

Cycle 10--Same reactants as used in cycle 30

Cycle 9--Same reactants as used in cycle 19

Cycle 8

Coupling: The resin peptide from cycle 9 was agitated for 10 minutes with 3.58 g. (0.0165 mole) of BOC-L-Valine and 25 ml. of methylene chloride. Then 8 ml. of DCCI in methylene chloride (equivalent to 0.0165 mole of DCCI) was added and the mixture agitated for 16 hours. The reaction mixture was removed by filtration. The resin peptide was subjected to two minute washes with two successive 25 ml. portions of the following solvents: methylene chloride, methyl alcohol, methylene chloride. Each individual wash was removed by filtration.

Deprotection: See cycle 31.

Cycle 7

The procedure was the same as used in cycle 31 except that in the coupling reaction 3.09 g. (0.011) of BOC-S-ethylthio-L-cysteine was used in place of the threonine derivative.

Cycle 6

The reactants and procedures used were the same as cycle 31.

Cycle 5

The reactants and procedures used were the same as cycle 29.

Cycle 4

The reactants and procedures used were the same as cycle 19.

Cycle 3

The reactants and procedures used were the same as cycle 26.

Cycle 2

The reactants and procedures used were the same as cycle 29.

Cycle 1

The reactants and procedures used were the same as cycle 31 except that 3.75 g. (0.011 mole) of BOC-S-p-methoxybenzyl-L-cysteine was used in place of the threonine derivative.

After completion of cycle 1, the resin peptide was washed with two successive 25 ml. protion of n-hexane. The peptide material was removed from the reactor and dried in an electric vacuum oven at 40 degrees centigrade and 0.1 mm. of Hg for 24 hours.

Cleavage With Hydrogen Fluoride

The dried resin peptide (2 g.) and 2 ml. of anisole were placed in a Teflon reaction vessel. The vessel equipped with a Teflon-coated magnet stirrer was placed in a dry ice-acetone bath and 15 ml. of hydrogen fluoride gas was condensed into the vessel. This mixture was stirred at 0 degrees centigrade in an ice bath for 1 hour. The hydrogen fluoride was removed by evaporation at reduced pressure. The residue was triturated with six 25 ml. portions of ethyl acetate. The peptide was extracted from the resin beads with 120 ml. of 0.1 molar aqueous acetic solution.

Cyclization Of The Peptide

The aqueous acetic acid extract obtained from hydrogen fluoride cleavage was diluted to 0.2 liters by addition of 80 ml. of distilled water. The pH of the solution was adjusted to 7.5 by the addition of concentrated ammonium hydroxide. The solution was stirred in a closed vessel under a stream of nitrogen for 24 hours. At this time no ethyl mercaptan could be detected in the emerging nitrogen stream. The ethyl mercaptan content of the nitrogen stream was measured by passing the stream through a solution of Ellman's reagent [Ellman, G. L., Arch. Biochem. Biophys., 82, 70-7 (1969)]. The pH of the reaction mixture was adjusted to 5.0 by addition of glacial acetic acid.

Purification Of The Crude Leu¹⁶ -SCT

The 0.2 liters of solution from the above synthesis at pH 5.0 was concentrated using a SP-Sephadex C-25 ion-exchange column. The 25 ml. concentrate removed from the column with 0.7 molar sodium chloride solution was desalted and purified by passing through a Sephadex G-25 (fine) gel-filtration column and eluting with 0.03 molar aqueous acetic acid solution. The des Leu¹⁶ -SCT fraction from this column was adjusted to pH 6.0 by addition of ammonium hydroxide solution. This solution was further purified by ion-exchange chromatography using a Whatman CM52 column eluted with ammonium acetate buffer. The Leu¹⁶ -SCT fraction from this column was adjusted to pH 5.0 by addition of glacial acetic acid. This solution was concentrated using a SP-Sephadex C-25 ion-exchange column. The 30 ml. concentrate removed from the column with 0.7 molar sodium chloride solution was desalted with a Sephadex G-25 (fine) gel-filtration column. The purified peptide fraction was collected and freeze-dried. The product was obtained as a fluffy white solid. Amino acid analysis of the product gave the following ratios with the theoretical values given in parenthesis: Asp 2.0 (2), Thr 4.9 (5), Ser 3.8 (4), Glu 3.1 (3), Pro 2.1 (2), Gly 2.9 (3), Leu 4.0 (4), His 0.9 (1), Lys 1.9 (2), Arg 0.9 (1), Tyr 0.85 (1). This product assayed at 580 MRC units per mg.

While only certain embodiments of our invention have been described in specific detail it will be apparent to those skilled in this art that many other specific embodiments may be practiced and many changes may be made, all within the spirit of the invention and the scope of the appended claims. 

What is claimed:
 1. A peptide of the structure: ##STR27##
 2. A peptide of the structure: ##STR28##
 3. A peptide of the structure: ##STR29##
 4. A peptide of the structure: ##STR30## wherein R₁ is S-n-alkyl, Cys or H and R₂ is S-n-alkyl or H, R₁ being S-n-alkyl, Cys or H when R₂ is H and R₂ being S-n-alkyl or H when R₁ is H and H when R₁ is S-n-alkyl or Cys, wherein the S-n-alkyl groups are selected from the group consisting of methylthio, ethylthio, n-propylthio, n-butylthio, and equivalents thereof.
 5. A peptide of the structure: ##STR31## wherein R₁ is S-n-alkyl, Cys or H and R₂ is S-n-alkyl or H, R₁ being S-n-alkyl, Cys or H when R₂ is H and R₂ being S-n-alkyl or H when R₁ is H and H when R₁ is S-n-alkyl or Cys, wherein the S-n-alkyl groups are selected from the group consisting of methylthio, ethylthio, n-propylthio, n-butylthio, and equivalents thereof.
 6. A peptide of the structure: ##STR32## wherein R₁ is Sn-alkyl, Cys or H and R₂ is S-n-alkyl or H, R₁ being S-n-alkyl, Cys or H when R₂ is H and R₂ being S-n-alkyl or H when R₁ is H and H when R₁ is S-n-alkyl or Cys, wherein the S-n-alkyl groups are selected from the group consisting of methylthio, ethylthio, n-propylthio, n-butylthio, and equivalents thereof. 